LTC3788
9
3788fc
Typical perForMance characTerisTics
pin FuncTions
SENSE1
–
, SENSE2
–
(Pin 1, Pin 9): Negative Current Sense
Comparator Input. The (–) input to the current comparator
is normally connected to the negative terminal of a cur-
rent sense resistor connected in series with the inductor.
The common mode voltage range on these pins is 2.5V
to 38V (abs max).
FREQ (Pin 2): The frequency control pin for the internal
VCO. Connecting the pin to GND forces the VCO to a fixed
low frequency of 350kHz. Connecting the pin to INTV
CC
forces the VCO to a fixed high frequency of 535kHz. The
frequency can be programmed from 50kHz to 900kHz
by connecting a resistor from the FREQ pin to GND. The
resistor and an internal 20µA source current create a volt-
age used by the internal oscillator to set the frequency.
Alternatively, this pin can be driven with a DC voltage to
vary the frequency of the internal oscillator.
PHASMD (Pin 3): This pin can be floated, tied to SGND, or
tied to INTV
CC
to program the phase relationship between
the rising edges of BG1 and BG2, as well as the phase
relationship between BG1 and CLKOUT.
CLKOUT (Pin 4): A Digital Output Used for Daisychaining
Multiple LTC3788 ICs
in Multiphase
Systems. The PHASMD
pin voltage controls the relationship between BG1, BG2
and CLKOUT. This pin swings between SGND and INTV
CC
.
PLLIN/MODE (Pin 5): External Synchronization Input
to Phase Detector and Forced Continuous Mode Input.
When an external clock is applied to this pin, it will force
the controller into forced continuous mode of operation
and the phase-locked loop will force the rising BG1 signal
to be synchronized with the rising edge of the external
clock. When not synchronizing to an external clock, this
input, which acts on both controllers, determines how the
LTC3788 operates at light loads. Pulling this pin to ground
selects Burst Mode operation. An internal 100k resistor
to ground also invokes Burst Mode operation when the
pin is floated. Tying this pin to INTV
CC
forces continuous
inductor current operation. Tying this pin to a voltage
greater than 1.2V and less than INTV
CC
– 1.3V selects
pulse-skipping operation. This can be done by adding a
100k resistor between the PLLIN/MODE pin and INTV
CC
.
SGND (Pin 6): Signal Ground. All small-signal components
and compensation components should connect to this
ground, which in turn connects to PGND at a single point.
RUN1, RUN2 (
Pin 7, Pin 8): Run Control Input. An external
resistor divider connects to V
IN
and sets the thresholds for
converter operation with a threshold of 1.28V. Once run-
ning, a 4.5µA current is sourced from the RUN pin allowing
the user to program hysteresis using the resistor values.
Maximum Current Sense
Threshold vs Duty Cycle
Charge Pump Charging Current
vs Operating Frequency
DUTY CYCLE (%)
0
MAXIMUM CURRENT SENSE VOLTAGE (mV)
80
100
70
60
40
20 40
10 90
30 50
80
60 100
20
0
3788 G25
I
LIM
= INTV
CC
I
LIM
= FLOAT
I
LIM
= GND
OPERATING FREQUENCY (kHz)
50
0
CHARGE PUMP CHARGING CURRENT (µA)
20
30
50
250 450 550
10
60
70
40
150 350 650 750
3788 G26
T = 130°C
T = 25°C
T = –45°C
V
SW
= 12V
V
BOOST
– V
SW
= 4.5V
